Excitatory effects of hypocretin-1 (orexin-A) in the trigeminal motor nucleus are reversed by NMDA antagonism

J Neurophysiol. 2003 May;89(5):2591-600. doi: 10.1152/jn.00968.2002. Epub 2003 Jan 15.


Hypocretin-1 and -2 (Hcrt-1 and -2, also called orexin-A and -B) are newly identified neuropeptides synthesized by hypothalamic neurons. Defects in the Hcrt system underlie the sleep disorder narcolepsy, which is characterized by sleep fragmentation and the involuntary loss of muscle tone called cataplexy. Hcrt neurons project to multiple brain regions including cranial and spinal motor nuclei. In vitro studies suggest that Hcrt application can modulate presynaptic glutamate release. Together these observations suggest that Hcrt can affect motor output and that glutamatergic processes may be involved. We addressed these issues in decerebrate cats by applying Hcrt-1 and -2 into the trigeminal motor nucleus to determine whether these ligands alter masseter muscle activity and by pretreating the trigeminal motor nucleus with a N-methyl-d-aspartate (NMDA) antagonist to determine if glutamatergic pathways are involved in the transduction of the Hcrt signal. We found that Hcrt-1 and -2 microinjections into the trigeminal motor nucleus increased ipsilateral masseter muscle tone in a dose-dependent manner. We also found that Hcrt application into the hypoglossal motor nucleus increases genioglossus muscle activity. Pretreatment with a NMDA antagonist (d-(-)-2-amino-phosphonovaleric acid) abolished the excitatory response of the masseter muscle to Hcrt-1 application; however, pretreatment with methysergide, a serotonin antagonist had no effect. These studies are the first to demonstrate that Hcrt causes the excitation of motoneurons and that functional NMDA receptors are required for this response. We suggest that Hcrt regulates motor control processes and that this regulation is mediated by glutamate release in the trigeminal motor nucleus.

Publication types

  • Research Support, Non-U.S. Gov't
  • Research Support, U.S. Gov't, Non-P.H.S.
  • Research Support, U.S. Gov't, P.H.S.

MeSH terms

  • 2-Amino-5-phosphonovalerate / administration & dosage
  • 2-Amino-5-phosphonovalerate / pharmacology
  • Animals
  • Carrier Proteins / administration & dosage
  • Carrier Proteins / antagonists & inhibitors*
  • Carrier Proteins / pharmacology*
  • Cats
  • Decerebrate State / physiopathology
  • Dose-Response Relationship, Drug
  • Electrodes, Implanted
  • Electromyography
  • Excitatory Amino Acid Antagonists / administration & dosage
  • Excitatory Amino Acid Antagonists / pharmacology
  • Female
  • Functional Laterality / physiology
  • Hypoglossal Nerve / cytology
  • Hypoglossal Nerve / physiology
  • Intracellular Signaling Peptides and Proteins*
  • Microinjections
  • Motor Neurons / drug effects*
  • Muscle Tonus / physiology
  • Muscle, Skeletal / cytology
  • Muscle, Skeletal / drug effects
  • Muscle, Skeletal / physiology
  • Neuropeptides / administration & dosage
  • Neuropeptides / antagonists & inhibitors*
  • Neuropeptides / pharmacology*
  • Orexin Receptors
  • Orexins
  • Receptors, G-Protein-Coupled
  • Receptors, N-Methyl-D-Aspartate / antagonists & inhibitors*
  • Receptors, Neuropeptide
  • Serotonin Antagonists / pharmacology
  • Trigeminal Nuclei / drug effects*


  • Carrier Proteins
  • Excitatory Amino Acid Antagonists
  • Intracellular Signaling Peptides and Proteins
  • Neuropeptides
  • Orexin Receptors
  • Orexins
  • Receptors, G-Protein-Coupled
  • Receptors, N-Methyl-D-Aspartate
  • Receptors, Neuropeptide
  • Serotonin Antagonists
  • 2-Amino-5-phosphonovalerate